Manafort was an insider, as you say, and as such, would actually never have been subject to any investigation - except that his association with Trump led them to hope they could get at Trump through him.
Your point is a good one about giving Trump a good response regarding his hiring, though. Maybe their secrecy and general all-over sneakiness will backfire on them. One can only hope...
Don’t count on your assumption. DOJ has revealed that Firtash was being wiretapped.
I am not 100% clear on when or if the 3 hop rule changed..or when Firtash was exactly being wiretapped...
Firtash to Manafort to Sater to Trump or other combos
The guy who worked in the office that is taking down Firtash parsed Cohen’s statement
https://www.justsecurity.org/45169/federal-prosecutor-dissects-trump-lawyer-michael-cohens-statement-senate/